The present invention generally relates to a method of manufacturing a polishing pad useful for polishing and planarizing substrates using a chemical-mechanical planarization (“CMP”) process. More particularly, the method of the present invention improves uniformity both within the pad and from one pad to another.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition, also known as sputtering, chemical vapor deposition, plasma-enhanced chemical vapor deposition, and electrochemical plating.
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials.
In a typical CMP process, a lower platen having a circular rotating plate holds a polishing pad; the polishing pad is attached such that the polishing surface of the polishing pad faces up. A polishing composition, which typically contains chemistry that interacts with the substrate and may contain abrasive particles, is supplied to the polishing surface of the polishing pad. An upper platen having a rotating carrier holds a substrate; the substrate is held such that the surface to be planarized faces down. The carrier is positioned so that its axis of rotation is parallel to and is offset from that of the polishing pad; additionally, the carrier can be oscillated or otherwise moved about the surface of the polishing pad as is appropriate for the CMP process. The substrate and the polishing pad are brought into contact and forced together with downward pressure by the upper platen, whereby the polishing composition on the surface of the polishing pad is contacted with the surface of the substrate (the working environment), causing the desired chemical reaction, and mechanical polishing takes place.
Optionally, the CMP process is continually monitored throughout in order to determine when the desired amount of material has been removed from the surface of the substrate. This is typically done by in-situ optical end-point detection that involves projecting laser light through an aperture or a window in the polishing pad from the platen side so that the laser light is reflected off the polished surface of the substrate and is measured by a detector. The amount of light that is reflected corresponds to the amount of material that has been removed from the surface of the substrate. When the amount of light detected equals a predetermined value, the CMP process has reached the desired end-point and the CMP process is terminated.
Polishing pads can be manufactured in a variety of ways, such as casting a cake or by casting a sheet. In a typical manufacturing process, the polymer pad material ingredients, which may include one or more pre-polymers, cross-linking agents, curing agents and abrasives, are mixed, resulting in a resin. The resin is transferred to a mold by pouring, pumping or injecting etc. The polymer typically sets quickly and may finally be transferred to an oven for completion of the curing process. The cured cakes or sheets are then cut to a desired thickness and shape.
Polishing pad surface asperities aid in transporting the polishing composition during the CMP process and can be created on the polishing surface of the polishing pad in many ways. According to one method, as disclosed in U.S. Pat. No. 5,578,362, surface asperities are created by embedding hollow polymeric capsules in a polishing pad comprising a polymeric matrix. Specifically, surface asperities are created by rupturing the capsules and exposing the hollow void contained therein to the working environment on the surface of the polishing pad. This is accomplished by conditioning the polishing pad.
Typically, conditioning consists of abrading the polishing surface of the polishing pad with diamond points (or other scoring or cutting means) embedded in the conditioning surface of a conditioning pad. As the conditioned polishing pad is used, the pores wear away and become clogged with debris from the CMP process. This results in the polishing pad losing surface asperities with use. Asperities can be regenerated as the polishing surface is worn during the CMP process, by continuous or intermittent conditioning. Asperities can also be regenerated without a conditioning pad as the embedded polymeric capsules are exposed and ruptured during polishing. For convenience, the term conditioning refers to regeneration of surface asperities whether through pad wear exposing new cavities, through the use of a conditioning pad or through other regeneration techniques.
Large scale texture is created on the polishing surface of the polishing pad by introduction of grooves. Groove pattern design and groove dimensions affect polishing pad characteristics and CMP process characteristics. Polishing pad grooving is well known in the art, and known groove designs include radial, circular, spiral, x-y and others. Typically, grooves are introduced in the polishing surface of a polishing pad after it is formed through mechanical means such as a straight blade like a chisel or other cutting means.
Polishing pads made according to the '362 patent, however, suffer from the tendency of the capsules to expand. The polymeric capsules expand during the curing process as they are heated by the exothermic curing reaction. The amount of expansion is difficult to control for two reasons. Expansion of the capsules due to heat is largely controlled by the ability of the shell to withstand the increasing pressure as temperature increases, which in turn depends on the shell thickness, among other things. The shells are typically very thin and so even a very small variation in shell thickness translates to a large percentage difference and a large relative difference in expansion.
The other factor that makes capsule expansion hard to control is the effect of differential heating. Differential heating occurs because the polymeric capsules act as thermal insulators, reducing the flow of heat from areas of higher temperature to areas of lower temperature. The areas of the cake or sheet close to the surface (those areas exposed to air or the mold) transfer heat to the surrounding environment and cool. The center of the cake or sheet, however, is insulated and the heat from the reaction builds up. The result is greater capsule expansion in the center of the mold than in the areas exposed to the air or the mold itself. Uneven expansion of the capsules results in non-uniform pad porosity, and therefore non-uniform pad density, which is disadvantageous. Therefore what is needed is a method of manufacturing a polishing pad that improves product uniformity and process consistency.